The production of copper foil by electrodeposition involves the use of an electroforming cell containing an anode, a cathode, an electrolyte solution and a source of current. The cathode is cylindrical in shape and the anode conforms to the curvature of the cathode to maintain a constant separation between the two. The electrolyte solution, which contains copper ions and sulfate ions, flows between the anode and the cathode. Voltage is applied between the anode and the cathode and copper deposits on the cathode. The copper feed stock, which is typically copper shot, copper wire or recycled copper, is dissolved in sulfufic acid to form the electrolyte solution. Various types of agents such as animal glue, thiourea and chloride ions are typically added to the electrolyte solution to control the foil properties.
Electrodeposited copper foil is supplied to PCB manufacturers in the form of laminates. These laminates are formed by bonding the copper foil to a polymeric insulating resin. The copper foil portion of the laminate is etched to form the conductive paths of the PCB using processes known to those skilled in the art. The etched copper conductive paths provide electrical connection between various portions of an electronic device.
To be acceptable for PCB applications, it is desirable that copper foils have controlled low profiles to provide etchability and impedance control. It is preferred that these foils have high ultimate tensile strengths (UTS) to provide desired handling and surface quality characteristics, and high elongations at elevated temperatures to resist cracking. Foils having high profiles result in laminates that exhibit measles and embedded copper. Foils having profiles that are too low result in laminates with insufficient bond strength. Foils with low elongations at elevated temperatures crack when temperature stressed. Foils with low UTS's wrinkle during handling.
The prior art suggests copper foils having many of these attributes, but there is a continuing demand for higher quality copper foils. By virtue of the inventive process, which involves the use of electrolyte solutions employing organic additives and critical concentrations of chloride ions below about 1 ppm, higher quality copper foils of the type that are acceptable for PCB applications are achieved.
Lakshmanan et al, "The Effect of Chloride Ion in the Electrowinning of Copper", Journal of Applied Electrochemistry 7 (1977) 81-90, discloses that the effect of chloride ion concentration on copper electrodeposition is dependent on the operating current density. At lower current density values the ridge type growth structure orientation is favored for additive-free electrolytes. At high current density values pyramidal growth orientation is favored for additive-free electrolytes. The addition of chloride ion to the 10 ppm level lowers the overvoltage and thus promotes ridge type oriented deposits. As the current density is increased to 40 amps per square foot [0.043 A/cm.sup.2 ], the pyramidal growth structure is again favored. The article indicates that the current densities that were tested ranged from 15 to 40 amps per square foot [0.016 to 0.043 A/cm.sup.2 ].
Anderson et at, "Tensile Properties of Acid Copper Electrodeposits", Journal of Applied Electrochemistry, 15 (1985) 631-637, discloses that the chloride ion concentration in an acid copper plating bath influences the ultimate tensile strength and elongation of the foil produced therefrom. The article indicates that at the current densities tested, acid copper plating baths require the presence of chloride ions to provide a ductile copper deposit. The current densities reported in the article ranged from 20 to 50 mA/cm.sup.2 [0.02 to 0.05 A/cm.sup.2 ]. Chloride ion concentrations in the range of 0 to 100 ppm are reported.
Kuwako et at, "A New Very Low Profile Electrodeposited Copper Foil," Printed Circuit World Convention 5, Technical Paper No. B 8/1, (1990), discloses an electrodeposited copper foil having a fine grain size, very low profile, high elongation at elevated temperature and high tensile strength over a wide temperature range. The reference indicates that this foil exhibits tensile strengths ranging from about 60 Kg/mm.sup.2 (85,320 psi) at about -50.degree. C. to 0.degree. C. to about 20 Kg/mm.sup.2 (28,440 psi) at about 200.degree. C. The elongation ranges from about 6% to about 10% over a temperature range of about -50.degree. C. to about 290.degree. C.
U.S. Pat. No. 2,475,974 discloses a process for making copper deposits having tensile strengths of about 60,000 to about 73,000 psi and elongations of 6% to 9% using a copper plating solution containing triethanolamine.
U.S. Pat. No. 2,482,354 discloses a process for making copper deposits having tensile strengths of about 65,000 to about 90,000 psi and elongations of 8% to 12% using a copper plating solution containing tri-isopropanolamine.
U.S. Pat. No. 4,956,053 discloses a process and apparatus for producing metal foil which comprises supplying an isotopically polished continuous cathode, a high volume turbulent flow of electrolyte and rigorous filtration to exclude chlorides, sulfides, organics and other impurities from the electrolyte. The foil so produced is free from micro-voids and able to withstand substantially higher biaxial stresses than conventional electrodeposited foil. The micro void free structure also decreases the diffusion ram of an underlying substrate.
U.S. Pat. No. 5,181,770 discloses an electrodeposition process for making copper foil using an electrolyte solution having a chloride ion concentration of either 0.5-2.5 ppm or 10-50 ppm. The reference indicates that all organic and inorganic additives as well as impurities are excluded from the electrolyte.
WO 91/19024 discloses electrodeposited copper foils having an elongation measured at 180.degree. C. in excess of about 5.5%, an ultimate tensile strength measured at 23.degree. C. in excess of about 60,000 psi, and a matte-side R.sub.tm in the range of about 4.5 to about 18 .mu.m. This reference also discloses a process for making electrodeposited copper foil which comprises: preparing a copper deposition bath comprising water, copper ions and sulfate ions, said bath containing less than about 20 ppm chloride ions; and applying electric current to said bath to electrodeposit copper from said bath using a current density in the range of about 200 to about 3000 amps per square foot [0.22-3.23 A/cm.sup.2 ].